The present invention relates generally to motion sensing systems for detecting motion on an apparatus caused by an external force. More particularly, it relates to a microelectromechanical system (MEMS) motion sensor for detecting movement of a portion of a semiconductor wafer system, by detecting a capacitance that varies with movement of the apparatus.
In the field of electronic measurement devices and other devices, it is desirable to determine when an apparatus is physically moved by an external force, or other force on the device. It is also desirable to determine the nature of the force, including such properties as the direction and strength of the force using a compact and inexpensive motion sensing device positioned on the apparatus. Various measurement systems detect forces exerted on a body. For example, it may be desirable to measure forces caused by earthquakes, by gravitational forces between stellar bodies, by vehicle movements, by persons acting on an object, or by any number of other sources. Systems exist for sensing such motion in one or two dimensions but cannot effectively sense motion in three dimensions.
Existing motion-sensing systems include systems used in accelerometers in vehicles and systems used in computer or video game input devices, such as joy sticks. Such systems sense motion in two dimensions using capacitance-based motion sensors that identify changes in movement based on changes in a capacitance. Opposing electrodes are positioned on a stationary portion of the device and a moveable portion of the device, and a capacitance is detected between the electrodes. The capacitance changes as the moving portion moves. Existing systems can sense movement in one or two dimensions using a single moving mass, but cannot sense movement in three dimensions using that same mass. Existing systems detect three-dimensional movement using multiple moving masses to create combinations of one-or two-dimensional motion sensors. This complicates the hardware design for determining the movement and consumes valuable space on the electronic device""s semiconductor wafer(s).
What is needed is a compact and inexpensive system for sensing movement of an apparatus in three dimensions. In particular, what is needed is a MEMS device for sensing three dimensional movement of an apparatus using a minimal amount of space and using a minimal number of moving parts.
A microelectromechanical system (MEMS) motion sensor is disclosed for detecting movement in three dimensions of a semiconductor wafer structure. The MEMS device has top, middle, and bottom layers, with a mover attached to the middle layer by a flexure that allows the mover to move in three dimensions relative to the layers. The system may be part of a semiconductor chip, such as a processor in an electronic device. The mover has mover electrodes that create a capacitance with counter electrodes positioned on an adjacent layer. The capacitance between the electrodes changes as the mover moves. A capacitance detector receives signals from each of the electrodes and detects movement of the mover based on the change in capacitances. The MEMS device processes the detected capacitances to determine the nature of the movement of the mover.
The mover and counter electrodes comprise x-y mover and counter electrodes for detecting movement in an x-y plane parallel to the middle layers, and z mover and counter electrodes for detecting movement in a direction orthogonal to the x-y plane. In one embodiment, the mover is connected to the middle layer by flexures that control movement of the mover by allowing the mover to move in three dimensions while urging the mover back to a static position. Each of the layers may be a separate semiconductor wafer.
A three-wafer MEMS device is also disclosed for detecting forces acting on the device. In the three-wafer device, each of the layers may be a separate semiconductor wafer, with the mover attached to the middle wafer. The MEMS device may have counter electrodes positioned on both the upper and lower wafers, together with corresponding mover electrodes.